Is it possible to change image texture after Render?
First, check version of blend4web:

Before 17.08:

//load modules:
var m_tex       = require("textures");
var m_scenes    = require("scenes");

//get object
var obj = m_scenes.get_object_by_name("_object_name_");

//change texture in object
m_tex.change_image(obj, "_texture_name_", "_image_URL_" );


After 17.08:

var m_scenes  = require("scenes");
var m_tex = require("textures");

var obj = m_scenes.get_object_by_name("_object_name_");
var image = new Image();
image.onload = function() {
    m_tex.replace_image(obj, "_texture_name_", image);
}
image.src = "_image_URL_";


Where:

_object_name_ is the name of your plane in Blender:



_texture_name_ is the name of texture data block  in Blender:



_image_URL_ url of your image in web.

More information:

https://www.blend4web.com/api_doc/module-textures.html#.replace_image

Live example:

https://www.blend4web.com/apps/code_snippets/code_snippets.html?scene=change_image